Simplify context managers for enabling units.
Fix doc warning
Add documentation for angles_dimensionless in units/equivalencies.rst
Add equivalencies to UnitRegistry, as well as add/set/context managers
Add documentation for the equivalency context manager
Use equivalencies=None, so that empty list can override global default
Adapt doctest to deal with small representation errors
Add comment to decompose to ensure no equivalencies are ever passed
Reinterpret equivalencies=[] and None; normalize on UnitRegistry input
Separate out context manager to bottom; add in-text references, cleanup
Cleanup after rebasing to new context manager; changes to equivalencies.rst
Doctest corrections
Add test for add/set_enabled_equivalency in nested contexts
Merge remote-tracking branch 'mhvk/units-angles-dimensionless-context'

Conflicts:
	astropy/units/core.py
	docs/units/standard_units.rst